I have my rigs set to -ethi 16
What does it actually do with a higher number? Without stating the obvious!
From what I can see it gives a better hashrate but thats about it.
Can't say for high numbers, but the resulting difference between -ethi 8 and -ethi 16 is the no of submitted shares and hence - effective pool hashrate. I recall 16-vs-8 intensity gives 5-7% more shares per hour.
Claymore said above -ethi 8 increased shares is minimal, but stale shares increases a lot, so it's not profitable to use higher than -ethi 8